In the development of gas turbine engines, it is often important to determine the amount of vibration of the rotating blades. From vibration measurements, stresses induced in the blades may be determined. Action can then be taken to avoid stresses which are high enough to cause damage to the blades.
It is known, for example, to mount strain gauges on rotating compressor/turbine blades to provide information about the amplitudes and frequencies of vibration of the blades. One or more strain gauges can be provided on each blade, and connected to a radio telemetry system mounted on the rotor, which transmits the measurements from the rotor. However, due to the number of strain gauges required to fully determine the vibrations, the telemetry system is typically complex, expensive, large and time-consuming to install within the rotor.
An alternative technique for characterising blade vibration is “blade tip timing” (BTT) in which non-contact timing probes, typically mounted on the engine casing, are used to measure the time at which a blade passes each probe. This time is compared with the time at which the blade would have passed the probe had it not been vibrating. This is termed the “expected arrival time” and can be calculated from the rotational position of the particular blade on the rotor in conjunction with a “once per revolution” or “OPR” signal which provides information about the position of the rotor. The OPR signal is derived from the time at which an indicator on the rotor passes a reference sensor, and its use is well known in the art.
The difference between the expected arrival time and the actual arrival time can be multiplied by the blade tip velocity to give the displacement of the blade from its expected position. Thus BTT data from a particular probe effectively measures blade tip displacement at the probe.
Typically, sets of BTT probes are located at both the leading and trailing edges of the blade. The relative axial position of the blade compared to the probes changes between the cold condition, in which the engine is built, and the hot running conditions. For this reason, during installation a margin is generally allowed between a set of BTT probes and the respective blade edge. This margin should be minimised to obtain the best quality data, but without risking the blade moving completely away from the probes such that their signals are lost.
It is sometimes the case, however, that a given casing design allows a set of BTT probes to be installed at only one of the blade edges. As a consequence, the quality of the analysis that can be performed using the BTT data is reduced because less is known about the precise location of the blade.